In both golf balls having a two-piece construction comprising a core and a cover and golf balls of multilayer construction comprising a core and two or more cover layers, the cover is generally injection-molded from a thermoplastic resin material.
The injection molding process is typically carried out, as shown in FIG. 4, using an injection mold 1 comprising upper and lower sections 2 and 3, both made of metal, which are removably mated to define a spherical cavity 4 at the interior.
Described below is one example relating to the formation of a cover around a core in the production of a two-piece golf ball. The cavity 4 within the mold 1 is defined by walls 7 on which are provided numerous projections (not shown) for embossing dimples on the surface of the cover, which is also the surface of the ball. The mold 1 has a parting line PL between the upper and lower halves 2 and 3 at a position corresponding to the equator H of the cavity 4. Referring to FIGS. 4 and 5, three core support pins 6 are disposed at the apexes of each of two equilateral triangles T centered respectively on the north and south poles of the spherical cavity 4. The core support pins 6 are extended into the cavity 4 in a direction perpendicular to the parting line PL of the mold 1, for supporting a core 5 at the center of the cavity. A gas venting pin hole 9 is provided in the mold at each of the two polar positions of the spherical cavity 4 and a gas venting pin 10 is received therein. These holes and pins are situated at the positions furtherest removed from cover stock injecting gates located near the mold parting line PL.
Injection molding is carried out in this mold by injecting the cover stock through the gates (not shown) located near the parting line PL. Just before injection of the cover stock ends, the core support pins are retracted so that the ends of the pins lie flush with the cavity wall. Injection molding is then completed.
During injection molding, the air present between the cavity wall 7 and the core 5 is vented through gaps between the gas venting pins 10 and the gas venting pin holes 9 and through gaps between the core support pins 6 and core support pin holes 6a, as seen from FIGS. 4 and 5. When this venting of gases or degassing takes place, some of the cover stock makes its way into the gaps, where it hardens, resulting in the formation of burrs at positions corresponding to the gaps.
Burrs that have formed locally on the surface of a golf ball in this manner can be removed in various ways. One such technique is a grinding method that makes use of a grinding apparatus comprising a plurality of shafts, each formed of a grindstone at the end face thereof (see JP-A 109880/1988).
This method grinds the surface of a golf ball G using a grinding apparatus 60 as shown in FIGS. 6 and 7. The apparatus 60 includes three shafts 62 disposed on the same plane at 120.degree. intervals so that the center axes of all three shafts meet at the center O of a golf ball G seated in a golf ball holder (not shown). Each shaft 62 is movable in its axial direction and rotatable about its own axis. Each shaft 62 has an end face 64 which is formed as a concave spherical surface of the same curvature as the surface of the golf ball G so that the face is engageable with the golf ball surface and hence, serves as a buffing face. The apparatus 60 further includes a mechanism for moving the shafts 62 in their respective axial directions and rotating the shafts about their respective axes. A disc-shaped recess 66 is provided at the center of each buffing face 64.
With this apparatus, grinding is carried out by pushing the shafts against the ball while rotating each shaft in the same direction. Grinding is performed while mutually varying the pressure against the ball and/or rotational speed of each of the three shafts. This causes the ball to turn during grinding, enabling the entire surface of the ball to be uniformly ground.
However, this prior-art method grinds not only those regions of the ball's surface having burrs, but even regions of the surface having no burrs. This undesirably compromises the flight performance of the ball.
In general, a golf ball is provided on its surface with numerous dimples to enhance flight performance of the ball. These dimples are usually formed to predetermined depths at a tolerance of within .+-.5 microns. However, if the entire surface of the ball is ground, including areas without burrs, the grinding operation destroys the precision that has been carefully imparted to the dimple-bearing surface of the ball. The resulting decline in the precision of the ball's surface, and especially in the precision of the dimple depths, causes the golf ball, when hit, to rise too sharply or to deviate right or left, which is evidence of a deterioration in the flight performance.